The additions of SiFz and Sic12 to ethylene have been studied using fourth order perturbation theory and the 6-31 1 Wd,p) basis set at geometries determined with second order perturbation theory. In contrast with earlier calculations on the analogous SiHz addition, found to proceed with no barrier, barrier heights of 18.4 and 4.5 kcallmol are predicted for the fluoro- and chlorosilylenes, respectively. These results are interepreted in terms of the net energetics, ring strain, and orbital properties.